The present invention concerns an inclinometer, in other words a device intended to measure inclination variations.
In particular, the invention can be used in mines, drillings, in the civil engineering field, for the surveillance of civil engineering structures (for example bridges and tunnels) and buildings (in particular, historical monuments), and anywhere where a precise control of the stability of a structure is required, particularly over the long term.
Diverse methods of auscultation are customarily used to monitor the amplitude and the rate of distortion, whether horizontal or vertical, of structures, of the surface of the ground and of accessible sub-soil parts in various categories of construction.
Customarily, in civil engineering, instruments called xe2x80x9cextensometersxe2x80x9d are used to monitor distortions. Various extensometers are commercially available.
As regards rotation measurements, electric inclinometers, also called xe2x80x9ctiltmetersxe2x80x9d are already known, which are used to monitor the change in inclination of points located on the ground or in the ground or placed on a structure. These inclinometers comprise a sensor that is sensitive to gravity (a pendulum) and arranged in an appropriate housing. Several types of inclinometers are known.
For example, a mechanical inclinometer is known, which comprises a beam and a bubble level, with an adjustment for levelling at one of its ends. This beam is fastened onto two reference spheres anchored on the system to be measured. The levelling adjustment is carried out by adjusting the bubble level and a tachometer dial is used to carry out the measurement. The measurement range is typically several degrees. The precision is approximately xc2x10.013 millimeters for a beam 200 millimeters long (i.e. around 60 xcexcrad) and is reduced to xc2x10.13 millimeters for a beam 900 millimeters long (i.e. around 150 xcexcrad).
An inclinometer comprising an accelerometric sensor is also known. The measurement is carried out by placing the inclinometer in a reproducible position on a flat reference piece. One takes a first reading then one turns the sensor by 180xc2x0 and one then takes a second reading. The flat reference piece is metallic or ceramic and must be securely fastened onto a surface that one wishes to control. The measurement range goes from xe2x88x9230xc2x0 to +30xc2x0 and the precision is typically xc2x1250 rad.
Moreover, an inclinometer comprising a pendulum fastened to the upper part of said inclinometer is also known. The inclination of the body of said inclinometer induces bending stresses on one piece. These bending stresses are monitored by two vibrating cord sensors fastened to each side of the piece. In another configuration, the two vibrating cord sensors are fastened between the pendulum and the cover of the inclinometer. This configuration makes it possible, thanks to a differential mounting, to disregard temperature effects.
The measurement range is typically from xe2x88x920.110 to +0.110 with a precision of around 0.5% of the full scale (around xc2x110 xcexcrad to xc2x1100 xcexcrad).
Furthermore, an inclinometer comprising an electrolytic level sensor is known. In a first embodiment, a glass measuring cell containing a liquid that conducts electricity (mercury for example) is sealed at its two ends. The measuring precision of this type of sensor is average. Moreover, its thermal sensitivity is high. As a result, it is not very suitable for geotechnical or civil engineering applications.
In a second embodiment, a vacuum is created in the measuring cell. The performance levels are better. Nevertheless, a specific calibration is necessary for each cell and re-calibrations are required. Such inclinometers are expensive, especially when they are used in a network.
We will now consider optic fibre sensors and particularly the advantages of such sensors, such as the insensitivity to electromagnetic perturbations.
First of all, it is worth recalling several facts concerning photo-induced fibre Bragg gratings.
A Bragg grating photo-induced in an optic fibre consists of a periodic structure formed by a modulation of the refraction index of the core of the optic fibre. This type of structure behaves practically like a mirror for a very fine spectral band around a characteristic wavelength xcexB (wavelength for which there is a phase tuning between the multiple reflections within the grating) and remains transparent to all other wavelengths. In fact, the multiple waves reflected at these other wavelengths are not in phase, interfere destructively in reflection and are thus transmitted.
The characteristic wavelength, called xe2x80x9cBragg wavelengthxe2x80x9d, is defined by Bragg""s law:
xcexB=2.Neff.xcex9
where xcex9 is the pitch of the effective index network neff.
The final characteristics of a photo-induced Bragg grating depend on the induction parameters such as the type of laser (wavelength, operating conditions) and the luminous intensity used, the wavelength xcex at which this network is induced, the effective index neff of the optic fibre in which it is induced, the amplitude of the modulation or variation of index An and the period xcex9 of said index variation.
These parameters determine the characteristic magnitudes of the Bragg grating, namely: the Bragg wavelength xcexB, the reflectivity Rmax at xcexB, and the width at mid-height of the reflectivity peak, as well as the propensity of the grating to withstand large temperature variations or considerable extensions, which is an important aspect for the use of said Bragg grating as a transducer.
We will now consider such a Bragg grating transducer. Given Bragg""s law that characterises this grating, the characteristic wavelength xcexB depends on the temperature and the stresses ("sgr"x, "sgr"y, "sgr"z) applied to the fibre in which the grating is formed.
It is normal to separate the three contributions, namely the temperature variations xcex94T, the extensions xcex5=xcex94L/L along the axis of the core of the fibre and the hydrostatic pressure variations xcex94P, according to the equation:
xcex94xcexB/xcexB=axe2x80x2.xcex94T+bxe2x80x2.xcex5+cxe2x80x2.xcex94P 
where axe2x80x2, bxe2x80x2 and cxe2x80x2 are coefficients that depend on the characteristics of the fibre and, to a lesser extent, on its temperature. In practice, they can be assimilated to constants, independent of the temperature, over a large range of operation.
Thus, a precise measurement of xcex94xcexB (variation of xcexB compared to an initial reference) makes it possible to determine the amplitude of the variation of the phenomenon that has induced this variation of xcexB. Beyond its simple role as a spectral filter, the Bragg grating thus constitutes a xe2x80x9ctransducerxe2x80x9d, since it transforms the changes in an influence quantity into a spectral shift proportional to these changes.
We will now consider the response of the Bragg grating to a temperature variation. When the grating is subjected to such a variation, it dilates or contracts, which modifies its pitch. Moreover, since the refractive index of a material depends also on the temperature, these two phenomena bring about a variation xcex94xcexB of the characteristic wavelength such that:             Δ      ⁢              xe2x80x83            ⁢              λ        B                    λ      B        =                    Δ        ⁢                  xe2x80x83                ⁢                  (                      n            ⁢                          xe2x80x83                        ⁢            Λ                    )                            n        ⁢                  xe2x80x83                ⁢        Λ              =                            [                                                    1                Λ                            ⁢                                                ⅆ                  Λ                                                  ⅆ                  T                                                      +                                          1                n                            ⁢                                                ⅆ                  n                                                  ⅆ                  T                                                              ]                ⁢        Δ        ⁢                  xe2x80x83                ⁢        T            =                                    a            xe2x80x2                    ·          Δ                ⁢                  xe2x80x83                ⁢        T            
In the case of silica, the coefficient axe2x80x2 is substantially equal to 7.8 10xe2x88x926/xc2x0 C.
By making a=axe2x80x2.xcexB, one can then state:
xcex94xcexB=a.xcex94T 
We will now consider the response of the Bragg grating to these distortions. As we have seen, stresses are also likely to modify the characteristic wavelength of the grating.
We can define the variation of the Bragg wavelength as a function of an extension as follows:             Δ      ⁢              xe2x80x83            ⁢              λ        B                    λ      B        =                    (                  1          -                                                    n                e                2                            2                        ⁢                          (                                                p                  11                                -                                  v                  ⁡                                      (                                                                  p                        11                                            +                                              p                        12                                                              )                                                              )                                      )            ⁢              ϵ        z              =                            (                      1            -                          p              e                                )                ·                  ϵ          z                    =                        b          xe2x80x2                ·                  ϵ          z                    
Where ne, xcex5z, E, xcexd and pe respectively represent the optical index of the core of the fibre, the relative variation of the length of the fibre (distortion along the axis z of the fibre), the Young modulus, the Poisson ratio and the photo-elastic constant of the material making up the fibre; p11, and p12 are opto-elastic coefficients. Taking the example of silica, the coefficient bxe2x80x2 equals around 0.78xc3x9710xe2x88x926/(xcexcm/m).
By making b=bxe2x80x2. xcexB, one can then state:
xcex94xcexB=b.xcex5z 
We will now consider the response of the Bragg network to a pressure variation xcex94P. The spectral response of the Bragg ray to this variation may be stated as: xcex94xcexB/xcexB=cxe2x80x2. xcex94P.
For silica, the coefficient cxe2x80x2 is equal to around xe2x88x922.87xc3x9710xe2x88x926/MPa.
The sensitivity values of photo-induced fibre Bragg gratings in silica optic fibres are shown in Table 1, for the most important parameters (temperature, distortions and pressure), for the three principal wavelengths used.
An optic fibre containing such Bragg gratings can thus be used as a distortion sensor. The measuring system for these distortions can then comprise a wide spectrum light source and a spectral analysis system (for example a spectrophotometer or Fabry-Perot type interferometric cavity) or a narrow scanning source (tunable laser type). The addition of reference Bragg gratings enables an absolute spectral positioning of the wavelength peaks that are reflected by the measuring Bragg gratings.
The aim of the present invention is an inclinometer that is insensitive to electromagnetic perturbations and which, in order to achieve this, uses one or a plurality of optic fibres as well as at least one Bragg grating as transducer.
Moreover, in the invention, spectral type measurements make it possible to disregard light intensity fluctuations.
More precisely, the aim of the present invention is an inclinometer intended to measure an inclination variation of a structure, said inclinometer being characterised in that is comprises:
an upper part intended to be rendered rigidly integral with the structure so that the inclination of said upper part varies like that of the structure,
a lower part intended to be located below the upper is part,
an articulation of the lower and upper parts, the lower part forming a pendulum freely suspended from the upper part by this articulation,
at least two portions of optic fibre placed on either side of said articulation, each portion of optic fibre having first and second ends which are respectively fastened to the lower and upper parts and which are previously put under tension between these lower and upper parts, and
at least one Bragg grating, this Bragg grating being formed in one of the two portions of the optic fibre, any variation in the inclination of the structure provoking a rotation of the upper part in relation to the lower part and inducing, as a result, a stress on the Bragg grating, said Bragg grating then being suited to modify a light that propagates in the portion of optic fibre where said Bragg grating is located, the variation in the inclination of the structure being determined from the light thereby modified.
According to a preferred embodiment, the inclinometer according to the invention comprises at least two Bragg gratings, said two Bragg gratings being respectively formed in the two portions of optic fibre and thus undergoing respectively a tensile stress and a compressive stress during the variation of inclination of the structure, said Bragg gratings making it possible to carry out a differential measurement of the wave length, the inclinometer then being insensitive to variations in ambient temperature and pressure.
The articulation of the lower and upper parts of the inclinometer according to the invention may comprise an axis of rotation intended to be arranged horizontally during the inclination variation measurement.
According to a specific embodiment of the inclinometer of the invention, the articulation of the lower and upper parts has at least two degrees of freedom of rotation, said inclinometer comprising at least three portions of optic fibre put under tension and placed around said articulation and at least two Bragg gratings, said two Bragg gratings being respectively formed in two of the three portions of optic fibre, the inclinometer then being provided for measuring an inclination variation definable by two angles of rotation. It is more advantageous to form a third Bragg grating in the third portion of optic fibre, in order to obtain a third measurement, used to compensate the effect of temperature.
In the case of this specific embodiment, the three portions of optic fibre may, for example, be arranged at 120xc2x0 in relation to each other around the articulation.
In this same case, the inclinometer may comprise four portions of optic fibre put under tension and arranged at 90xc2x0 in relation to each other around the articulation, and in addition at least two Bragg gratings, said two Bragg gratings being respectively formed in two of the four portions of optic fibre, and making it possible to measure respectively the two angles.
In this latter case, the inclinometer can comprise four Bragg gratings which are respectively formed in the four portions of optic fibre, each Bragg grating being associated with the Bragg grating that is opposite to it in relation to the articulation, said associated Bragg gratings enabling a differential measurement of the wavelength, the inclinometer then being insensitive to variations in ambient temperature and pressure.
The articulation having at least two degrees of freedom of rotation can comprise a spherical joint or a Cardan type suspension, or even a point pressing on a hard surface.
The inclinometer according to the invention can comprise a single optic fibre to which belongs each portion of optic fibre.
In this case, one can use a plurality of inclinometers according to the invention, these inclinometers being assembled in series by the intermediary of the optic fibre.
The inclinometer according to the invention can, in one specific embodiment, be interrogated in reflection, by at least one of the ends of the optic fibre in which is or are formed the Bragg grating(s).